1. Field of the Disclosure
The invention relates to high power fiber laser systems. Particularly, the invention relates to a fiber combiner configured with a plurality of SM fiber lasers which collectively radiate a combiner output characterized by minimally possible M2 factor.
2. Discussion of the Known Art
The most significant keys to ensuring broad marketability of fiber lasers is the development of producing ever-increasing laser output powers without sacrificing beam quality. Known for its high-quality beam, a single-mode (SM) fiber laser may deliver output powers approaching ten kilowatts, such as SM fiber lasers developed and manufactured by IPG Photonics Corporation, Oxford, Mass., U.S.A. Despite the impressive results, even higher power levels and beam quality are required for current and future industrial and military needs.
It is known to power scale laser output by combining the outputs from several SM fiber lasers while sacrificing beam quality as the power increases. The combined single modes of respective fibers translate into a multimode (MM) combiner output. The beam quality may be characterized by an M2 factor. The lower the M2 factor, the higher the beam quality. In a diffraction-limited Gaussian beam, the M2 factor is as low as one. The diffraction-limited beam is manifested by a single light spot. In some applications, the beam quality factor is not critical, in others, which are of interest here, it is.
FIG. 1 illustrates the concept of divergence of a Gaussian beam radiated by SM fiber. Quantitatively, the far-field divergence of the SM beam can be measured as
                    θ        =                              2            ⁢            λ                                π            ⁢                                                  ⁢                          MF              D                                                          (        1        )            Wherein θ—divergence half-angle, MFD—mode field diameter. In single mode fibers, the half angle is correlated to a numerical aperture as follows:sin θ=NA  (2)Accordingly, the divergence of the SM beam can be controlled by changing the MFD, which is the waist diameter of the Gaussian beam in the SM fibers (FIG. 1).
FIG. 2, related to the above-disclosed equations, illustrates the dependence between the divergence or numerical aperture (“NA”) and MFD. The greater the MFD, the smaller the divergence. The MFD, in turn, depends on a core size Dc of SM fiber, as can be seen in FIG. 3. Seemingly, the limitless decrease of the core diameter causes the increase of the MFD. In reality, the MFD cannot be limitlessly increased without detrimentally affecting the beam quality, as discussed below.
FIGS. 4 and 5 illustrate a known fiber laser system 10 configured with combined multiple SM laser outputs 12 which are placed next to one another to form a combiner 14 with an output beam 16. Disclosed only as an example, seven parallel outputs of respective SM fiber lasers 12, each having a 125 μm outer diameter Df, define an effective area X of output light beam 16 with a 375 μm overall outer diameter Db.
Since SM fibers 12 do not experience external stresses, beams 18, propagating in respective cores 20, each have a Gaussian shape. The MFD of each propagating beam is relatively small, and therefore, a far field divergence thereof is broad. Superimposed with one another, seven distinct and spaced apart fiber outputs define combiner output 16 with a large effective area X. The large effective area of the beam, i.e. an imaginary boundary running around cladding of respective fibers 12, represents a broad far-field divergence or small numerical aperture and, therefore, a high M2 factor of beam 16. In other words, system 10 radiates a low-quality, relatively unfocused combiner output.
The fiber laser system 10 not only radiates an output beam of poor quality, but also the system is labor- and cost-ineffective. After the combiner output diverges along seven fiber output paths, each fiber output is associated with a bulk optics (telescope) located along the fiber path. Only then seven fiber outputs converge toward one another. The necessity of seven additional bulk optic units adds labor efforts to the manufacturing and tuning process and, therefore, may make the combiner prohibitively expansive.
In practice, the terminal ends of respective fiber lasers 12, defining combiner 14, are often processed to reduce the effective area of output beam 16. However, as far as Applicants know, there are no established methods of controllably reducing the far-field divergence and M2 factor for active or doped SM fibers. The “blind” minimization of the effective area of the combiner beam, however, may not lead to satisfactory results, such as a low M2 factor and small divergence of combiner output, as discussed hereinbelow.
A need, therefore, exists for a method of controllably manufacturing a fiber combiner operative to output a high power beam characterized by narrow far-field divergence or small numerical aperture (NA) and low M2 factor.
A further need exists for a high power laser system with the disclosed combiner.